Power feed equipment is required to provide electrical power to the cables of submarine communications systems. An example of a conventional power feed is shown schematically in FIG. 1. The power feed 1 is constructed using a stack of four identical power converters 2.sub.1 to 2.sub.4 connected in series which together convert a 50V DC supply at the input terminals of the power feed to provide a 10 kV DC power feed voltage across the output terminals. The first power converter 2.sub.1 in the stack provides a 2.5 kV DC output, but it is elevated from earth by an additional 7.5 kV DC due to the effect of the adjacent power converters 2.sub.2 to 2.sub.4 in the stack. The centre-tapped transformer secondary winding 3 of the transformer 4 in the first power converter 2.sub.1 must therefore be insulated to 10 kV DC from earth to prevent Corona discharge and ultimately flash-over. Accordingly, blocking capacitors 5 and 6 are provided in this circuit which are designed to block the high DC voltage relative to earth and thereby insulate the secondary winding. Due to the magnitude of the DC voltage, the required blocking capacitors are large, bulky and expensive, and are not available as off-the-shelf components. Indeed, each capacitor typically has a capacitance of 0.5 .mu.F and a DC voltage breakdown greater than 15 kV. The use of such components is clearly undesirable.
Furthermore, although typically a nominal 2.5 kV DC output is produced by each power converter the corresponding AC pulse voltage from the high voltage secondary winding 3 is about 4 kV to allow for pulse width voltage control. Accordingly, rectifying and smoothing circuits 7 associated with each power converter must be designed and built using electrical components which can withstand these high AC voltages. Again, as with the DC blocking capacitors, such components are expensive and not generally available off-the-shelf.